Having a system to monitor and determine a particle flow in devices which use abrasive materials in a process stream, and which minimizes the maintenance and calibration of such a device, while providing process repeatability is highly desirable.
This invention relates to monitoring and determining a particle flow in a process stream. In particular, it is for use with a micro abrasive blasting machine. More particularly, the invention relates to a method and apparatus for opto-electronically determining a flow of abrasive particles in such a machine.
Of the various particle flow monitoring techniques in use in the industry, only a few address the measurement of particle flow in a process stream. The prior optical techniques rely on measurement of the optical density, scatter, or reflectivity of the particles in the process stream. All of these techniques have difficulty, at low particle densities, with contamination of the optical surfaces exposed to the process stream. Where a known zero particle flow condition can be achieved, it is possible to overcome this difficulty by frequent calibration or "zero setting." In many cases, however, calibration is inconvenient or impossible.
One of the prior techniques measures and calibrates the capacitance changes due to the particle content of the stream. However, this technique works better at high stream densities, because maintaining accuracy at very low densities is difficult. Other prior techniques employ acoustic damping or backscatter measurements, but these methods are more suited to large size streams. Yet other of the prior techniques detect and quantify particle impacts on a probe with acoustic or electronic detectors. In small high velocity abrasive streams, however, the probes are subject to rapid damage, which results in frequent maintenance. Accordingly, there is a need to provide a method and apparatus which minimizes these difficulties.